I've installed a program named Tag Runner. It can be used for basic tags, cover art and / or lyrics. I use it only with the "lyrics" option activated (Don't want it to mess with my other tags).
It's not freeware, but there is a trial period and then a small fee.
The main advantage is that it finds lyrics for many songs, even very new ones. The main disadvantage is that it does not have an MC plugin to work with so you have to manage your files outside MC.
This is what I did... I spent a couple of weeks passing all my 338,000 songs through the program, in small batches. Then, I performed an autoupdate in MC for external changes.
Now, before importing new songs into MC I run Tag Runner on them to find the lyrics, and then import the songs into MC (with the lyrics already inserted).
Every month or so, I let again run all the songs imported in the last weeks through Tag Runner to let it find some more lyrics (mainly of those songs that were too recent when they were imported for the first time).
I plan to run a whole library pass through Tag Runner once a year or so.
Almost 150,000 songs with lyrics in my library now (out of a total of 338,000 including instrumentals). I'd say that 70 or 80,000 have been filled with TagRunner... lyrics not available in Datamaster or Evyl Lyrics, that were the tools I used before.
Not the perfect solution but works for me until something better comes up.
[EDIT] In case you try it, I suggest to disable the "SeekLyrics" source... it was giving bad results and I'm not sure whether it has been already fixed or not. The program updates sources on a weekly basis.